Here?s some basic info on fertilizer and nutrients that plants require. The 3 numbers on a fertilizer bag refer to the percentage of N (nitrogen), P (phosphorus), and K (potassium) in the bag. There are different formulations for different purposes. In general terms, nitrogen produces lush green growth, phosphorus helps strengthen stems and produce flowers, and potassium keeps the root system healthy. If you're applying fertilizer to fruiting (e.g., tomatoes) or flowering plants, you're not as interested in the plant developing leaves as you are in it flowers and fruit, so you'd use a formulation lower in nitrogen and higher in phosphorus such as 5-10-5.

Plants in containers generally require more frequent fertilizing than those in the ground although they usually don't need to be fed during dormancy. Start applying fertilizer when new spring growth appears. About once per month, let water drain out of the pot, leaching away excess salts that accumulate from chemical fertilizers.